The field of the invention is pore-forming synthetic resins from foamed or foamable beads. The invention is particularly concerned with fine particle expandable styrene polymers having the surface of the particles coated with betaines.
The state of the art of foamed polystyrene may be ascertained by reference to the Kirk-Othmer, "Encyclopedia of Chemical Technology," 2nd Edition, Vol. 9 (1966) under the section entitled "Foamed Plastics," pages 847-844 and Vol. 19 (1969) under the section entitled "Styrene Plastics," pages 85-134, particularly pages 116-120 where polystyrene foams are disclosed, and U.S. Pat. Nos. 3,389,097; 3,503,905; 3,682,844; 3,789,028; 3,972,843 and 4,020,022, the disclosures of which are incorporated herein.
Betaines are disclosed in Kirk-Othmer, ibid. Vol. 5 (1964) p. 405 and Vol. 6 (1965) page 592, USSR Pat. No. 339,556, as reported at Chemical Abstracts Vol. 77, page 102670a and Japanese Pat. No. 56411 (1976), as reported at Chemical Abstracts Vol. 85, page 159449e, the disclosures of which are incorporated herein.
Small particulate expandable styrene polymer based molding compositions are advantageously obtained by suspension polymerization of the monomer in the presence of gaseous or liquid blowing agents. The suspension polymers containing the blowing agents are obtained in the form of beads with diameters between 0.1 and 4 mm. Beads of a very specific range in size are required when foams are made for the various applications. Thus, beads of small diameter are used to make thin-walled foams, while beads of larger diameters are used for insulating foam blocks.
Beads of a specific size are obtained by sifting production runs. This is conventionally done so that the beads separated from the aqueous phase are washed and dried and then placed on moving screens. Problems are encountered with strong electrical charges generated by the motion and the mutual friction of the beads on the screen. This seriously hampers sifting because the smaller beads cling like grapes to the larger ones and therefore fail to come in contact with the sifting area. Agglomerated particles may clog the screen meshes, whereby the sifting effectiveness is reduced.
The expandable molding compositions are processed according to the state of the art first by being prefoamed by heating in water or steam and then being foamed further in molds and thus sintered into molded bodies. A substantial proportion of fine particles, that is, the proportion of beads with diameters less than 0.1 mm will particularly interfere in the individual fractions because the steam intake nozzles of the processing equipment are clogged by these fine beads. Following drying and intermediary storage, the pre-foamed molding materials are pneumatically conveyed to the final foaming equipment. Here again, electrostatic charging takes place, agglomerates are formed and line clogging occurs in the conveyance which interferes with the regular procedure.
Accordingly, there have been many attempts in the past to improve the sifting by means of additives (sifting accessories).
Australian Pat. No. 66/5068 and French Pat. No. 1,478,998 describe the addition of a quaternary ammonium compound. These coatings are found to be impractical because the amines arising from the inevitable dissociation reaction of the quaternary ammonium compounds result in odor pollution in processing and application. Furthermore, quaternary ammonium compounds may be physiologically objectionable and may cause eczema.
West German Published Application 16 69 746 discloses coating fine particulate, expandable molding compositions containing a styrene polymer and a blowing agent with a sulfonium salt of which the cation comprises a hydrocarbon group with 6 to 24 carbon atoms. Such substances very substantially lower the boundary surface tension of styrene and water, which is particularly adverse where the sifted fine particulate proportion will be used again dissolved in styrene in the next polymerization batch. Because of the large surface of the proportion of the fine particles, the subsequent attempt at polymerization is supplied with non-negligible amounts of the boundary-surface active substance, and the suspension polymerization is interfered with. The boundary-surface active substance cannot be separated from the proportion of fine particles even after multiple rinsing, so that this proportion cannot be made use of in further inputs.
German Published Application 26 25 389 discloses a process for improved grading, where the expandable beads obtained after polymerization are treated with an aqueous solution of a hygroscopic metallic salt. This method suffers from the drawback that the metal salts used cause strong corrosion in metal surfaces, and therefore this method cannot be used over extended periods of time without damaging the production equipment.